Cyclic imide compounds, and in particular N-Hydroxyphthalimide (NHPI) have many potential uses. In particular, they have shown promise as radical mediators in a number of radical based oxidation reactions, such that these compounds can be used to catalyze oxidation reactions of hydrocarbons. For instance, cyclic imides such as NHPI are useful in the oxidation of cyclohexane to cyclohexanone and/or cyclohexanone. Another example is the oxidation of alkylbenzenes (such as, e.g., cyclohexylbenzene, also referred to herein as “CHB”) to corresponding alkylbenzene-hydroperoxides (in particular, oxidation of cyclohexylbenzene to cyclohexyl-1-phenyl-1-hydroperoxide, referred to herein as “cyclohexylbenzene-hydroperoxide” or “CHB-HP”). As described previously (e.g., in US 2014/0148569, US 2013/0211036, and US 2013/0203984), NHPI-catalyzed oxidation of cyclohexylbenzene is particularly advantaged in a process for making cyclohexanone from benzene via: (i) hydroalkylation of the benzene to cyclohexylbenzene; (ii) oxidation of the cyclohexylbenzene to cyclohexylbenzene-hydroperoxide; and (iii) cleavage of the cyclohexylbenzene-hydroperoxide to phenol and cyclohexanone.
Oxidation of hydrocarbons (e.g., cyclohexane and/or alkylbenzenes) may preferably be a gas-liquid oxidation that takes place through a free radical chain reaction homogeneously catalyzed by the cyclic imide (e.g., NHPI), for instance as described in WO 2014/137623 with respect to the alkylbenzene cyclohexylbenzene. In particular, the liquid-phase reaction medium comprising the cyclohexylbenzene is contacted with an oxygen-containing gas (e.g., air or O2) to form the CHB-HP. However, many of the most suitable cyclic imides to catalyze the free radical chain reaction, such as NHPI, are solids.
Cyclic imides such as NHPI have also been reported as useful in the oxidation of cyclohexanol to cyclohexanone; and/or the oxidation of cyclohexane to cyclohexanol and/or cyclohexanone. See, e.g., Huang et al., “Catalytic performance of N-hydroxyphthalimide-immobilized cross-linked polystyrene microspheres in oxidation of toluene and cyclohexanol,” The Chinese Journal of Process Engineering 14(4):683-688 (August 2014); Guha et al., “Aerobic Oxidation of Cyclohexane using N-Hydroxyphthalimide Bearing Fluoroalkyl Chains,” Adv. Synthesis & Catalysis 350(9): 1323-1330 (June 2008).
What is needed, therefore, is a mechanism for the satisfactory delivery of solid cyclic imide to the oxidation reaction that is suitable for relatively high throughput rates desired in an industrial-scale oxidation reaction.
Some further references of potential interest in this regard may include: U.S. Pat. Nos. 4,956,168, 5,472,679, 6,299,734, 6,316,639, 7,396,519, and 7,582,774; EP Patent Publication 108294 A, German patent publications DE-A-1247282, DE-A-3528463, and DE-A-3601803; Japanese patent publications JP 2001-233854, JP 2002-047270, JP 2002-128760, JP 2003-081941, and JP 2004-051626; Chinese patent publications CN1051170, CN101845012; and WIPO Publication Nos. WO 95/25090, WO 97/22551, WO 2014/137623, and WO 2016/053583. None of these references addresses the need for efficient, safe, and readily controlled delivery of solid cyclic imides to liquid and/or gas-phase oxidation reactions at an industrial scale.